The present invention relates, in general, to electronics, and more particularly, to methods of forming semiconductor devices and structure.
In the past, the semiconductor industry utilized various structures and methods for producing bus hold circuits. Bus hold circuits generally were used in applications where multiple circuits were connected to a single bus that provided signal interconnect between the multiple circuits. In some cases, the driver circuits that drove the bus entered a tri-state mode which allowed the bus to float. The bus hold circuits maintained the bus at the level of the last signal that was applied to the bus. FIG. 1 illustrates a schematic of a prior bus hold circuit 100. Bus hold circuit 100 included a first inverter 103 and a second inverter 104 connected in a back-to-back latch configuration so that the output of inverter 103 was connected to the input of inverter 104 and the output of inverter 104 was connected to the input of inverter 103. Consequently, when an external signal was applied to an input 101, an output 102 was forced low and the state was latched by inverter 104. Inverter 103 operated directly from a source of operating power that was applied to a power supply input 106 of circuit 100. A diode connected transistor 105 typically was used to connect a power input terminal 107 of inverter 104 to power supply input 106. When the external signal applied to input 101 was removed, circuit 100 typically could only pull input 101 to a value that was approximately equal to the value of the operating voltage minus the threshold voltage of transistor 105. In some cases, the lowered output voltage value was too low to be recognized as a logic high thereby causing inaccurate system operation.
In some embodiments, a Schottky diode replaced transistor 105 in order to lower the forward voltage drop. The Schottky diode operated the same as diode connected transistor 105 but at the lower voltage drop and had the same disadvantages including the lowered output voltage value. Process to implement Schottky diodes typically were most costly than other processes thereby increasing the cost of the devices using the Schottky diodes.
Accordingly, it is desirable to have a bus hold circuit that does not lower the maximum value of the output voltage of the bus hold circuit, that facilitates the input voltage having a value that is greater than the operating voltage of the device, and that protects the circuit from input voltage values that are no less than the operating voltage.
For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-Channel devices, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with the present invention.